1100 SUMMARY OF CUEEENT EE8EAECHES EELATING TO 



especially adapted for protecting mounts in his new stannous chloride 

 mounting medium.* It is made by diluting a somewhat thick shellac 

 cement with benzole, and adding sufficient litharge to give a con- 

 sistency about the same as that of white zinc cement. It dries very 

 quickly, forms a much harder ring than does the white zinc cement, 

 and is not unpleasant in appearance, as it becomes quite brown or 

 dark on exposure. A thin coat should first be applied, and when this 

 is well dried it should be followed by another. So far as tried this 

 seems to promise better than any other for preservation of the stannous 

 chloride mounts. The white zinc often fails, and while the wax 

 rings appear to answer admirably, the cement is more readily applied, 

 and if the future use of it confirms the present promise it will be more 

 acceptable. 



Dry Mounting. — The ordinary method of fastening on the cover- 

 glass is, in Mr. J. L. W. Miles's opinion, | the cause of a serious defect 

 in most dry mounts, viz. imprisoned moisture on the under side of 

 the cover. With very low powers it is not always noticeable, but 

 with 1 in., 1/2 in., or 4/10 in. objectives definition is seriously impaired. 

 It is usual to put the slide on the turntable and apply brown or other 

 cement freely to the rim of the cell, to which the cover-glass adheres 

 when placed thereon. The cement drying from the outside, the im- 

 prisoned portion upon which the cover rests hardens by evaporation 

 within the cell, hence the result mentioned. This difficulty can be 

 minimized, and in many cases, with care, entirely overcome by pro- 

 ceeding as follows : — Select a cover-glass much less in diameter than 

 the cell is, measured across its outer edges ; place and hold in posi- 

 tion with a wire clip, and unite the edge of the glass to the rim of the 

 cell by means of " tacky " gum, which should not run under, or but 

 slightly, inasmuch as the cover-glass will not overlap the cell rim, 

 but will barely rest upon its inner edge. There is yet another pre- 

 caution to be taken, namely, file out a small portion of the cell, which 

 will form an orifice or opening after the cover is put on. This is a 

 capital plan when you are in doubt about the dryness of your object, 

 as the minute opening can be plugged or bridged over by cement at 

 any convenient time afterwards. Having got so far, all difficulties 

 would appear to be overcome, but this is not so. It is necessary to 

 carefully finish the slide with varnish or cement of a damp-resisting 

 nature. Use brown cement in the first place, and finish with white 

 zinc, which clings tenaciously to clean glass, and makes a secure and 

 neat finish. 



Mr. T. W. Lofthouse,J in regard to moisture getting into card- 

 board cells, considered that if the cell was not entirely coated with 

 cement the moisture would be able to escape at the sides, and tested 

 this by mounting two slides with a drop of water on the under surface 

 of the cover before cementing it down. On warming the slide the 

 cell was soon completely filled with moisture. After being held over 



* See the preceding uote. 



+ Trans, and Ann. Eep. Manchester Micr. Soc, 1884-5, pp. 26-9. 



X Ibid., pp. 32-3. 



